Low Pressure Carbon Dioxide Inerting Fire System - Coal (Silo)Bunker
Carbon dioxide is one of most efficient and cost effective agents used by the aim of suppression and inerting for more than 80 years. The Carbon dioxide gas has a high rate of expansion and is applicable to a wide range of fire hazards from the systems first developed in the early 1900. The NFPA standard for fire carbon dioxide extinguishing systems was initiated in 1929 which is one of the oldest code of suppression system.
Carbon dioxide a colorless, odorless, and chemically inert gas that is both readily available at relative low cost and it is electrically non-conductive. The extinguishing effect is mainly due to the reduction of the oxygen concentration in combination with a certain degree of cooling. Carbon dioxide vapor chokes off combustion quickly. The dry ice “snow” in the discharge allows “local application” protection of non-enclosed hazards. It is the only gaseous agent that capable to supress fire non-enclosed hazards allowed by NFPA.
Carbon dioxide systems are divided in to two groups, being low pressure carbon dioxide systems and high pressure carbon dioxide systems. The choice for either one depends on the volume(s) to be protected and specific design requirements. A low pressure carbon dioxide fire system is generally used for applications that need a large quantity of agent storage rather than high pressure carbon dioxide systems.
A silo is a structure for storing bulk materials. Silos are more commonly used for bulk storage of grain, coal, cement, carbon black, woodchips, food products and sawdust. Three types of silos are in widespread use today: tower silos, bunker silos, and bag silos. 
Fire occurs from fuel, ignition and oxygen, sides of fire triangle. As known, fire cannot survive without one of them. Furthermore, two more elements, confinement and dispersion, need to be added to the fire triangle to create the Dust Explosion Pentagon. Much like with the fire triangle, taking away even one of these elements can remove the risk of a dust explosion; however the risk for a fire can still be present.
There is always a potential risk of explosions when handling combustible bulk solids and powders. Moreover, sudden spontaneous combustion and explosion risks lurk in every stage of the storage, processing and transportation of combustible powders.
Oxygen and fuel cannot be avoided in a coal‐ fired power plant, but the heat source can originate from several different sources. A common cause is the conveyor belt. As the coal is being transported from storage to use, the coal‐dust begins to fall off the belt and accumulate. Once the dust accumulates to 1/32 of an inch, or about the breadth to leave a footprint, it becomes a fire hazard. NFPA 654 defines combustible dust as, “any finely divided solid material that is 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in the air.” If a conveyor belt is not in impeccable condition, and one moving part stops, the friction can create a heat source for combustion. Other causes of heat are friction through mixing operation, electrical shortage, tool usage, or storage bin transfer.
Moreover, fire of silos is tend to have hot spots deep inside and that is why an inerting system which avoid dust explosions and smouldering fires in silos, by creating an inert atmosphere, must be provided for coal silos. NFPA 12 code cannot be applied for these system because this design is not be considered as fire suppression system. NFPA Handbook “Section 7. Storage and Handling of Solid Fuels - Chapter 1. Storage and Handling of Solid Fuels” must be applied for this system. The design basis mentioned below.
Carbon dioxide vapor has proved to be an effective fire- suppression agent when forced through the coal by pressure injection in the upper and lower parts of the silo. The rate of application should be sufficient to compensate for absorption by the coal. Inerting can normally be accomplished within an 8-hr shift, if an adequate supply of carbon dioxide is available.
Carbon dioxide inerting is generally considered successful when the carbon dioxide concentration reaches 65 percent. However, the logistics of injecting carbon dioxide might result in almost 100 percent saturation throughout most of the silo in order to achieve at least 65 percent concentration in all portions of the silo. Since leakage in the silo is inevitable, it will be necessary to anticipate using a large quantity of carbon dioxide. It is advisable to order a quantity of carbon dioxide equivalent to about three times the gross volume of the silo with 8 ft³/lb (2 kg/m³) discharge rate when using a low pressure system.
Carbon dioxide is stored as liquid inside of tank and it must be vaporized before injection into silo. This is accomplished using vaporizer sized such that the anticipated application rate. Application rate depends on hourly injection rate. It can be increased to anticipated leakage rate of coal silo.
As known, high pressure carbon dioxide have been using for coal silos and inject large amount of carbon dioxide inside on typical application for fire suppression. Injection of large amount carbon dioxide agent in short time causes the liquid to freeze into a solid that blocks the passage because of hard environment inside of coal silo. Moreover because of high pressure there must be occur kick back pressure on pipe and / or damage pipe or silo.
Basic calculation for 500 cubic meter coal silo is mentioned below:
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